provide food for a colony of flamingos. Tube-shaped microbialites have also been reported. A smaller lake known as Laguna Pabellon lies just south of Laguna Diamante. North of the resurgent dome, the Rio Aguas Calientes drains the caldera northward, while east of it the Rio Leon Muerto runs eastward out of the caldera. Rivers in the caldera and neighbourhood display river terraces which may reflect pre-caldera formation uplift of the terrain and uplift associated with the resurgent dome. These drainages eventually converge in the Rio de Los Patos and end into the Salar del Hombre Muerto north of Galán. The western flanks of the caldera drain into the Antofagasta de la Sierra valley through a number of drainages such as Rio Punilla, Rio Toconquis, Rio Miriguaca, Rio Las Pitas; the waters eventually end into the Laguna Antofagasta south of Antofagasta de la Sierra. Two hot springs are found within the caldera, the first close to its northern end and the second on the southwestern foot of the resurgent dome, both emitting water with temperatures of about . The first one is known as the Aguas Calientes hydrothermal spring and features deposits of tufa and boiling water. Another geothermal system is known as La Colcha and includes fumaroles as well as boiling water and sinter deposits; it has been prospected for the possibility of geothermal power generation. Geology The basement beneath the caldera consists of 600–365 million years old metamorphic and sedimentary rocks of Precambrian to Paleozoic age. These include intrusions of granitoid character and are overlain with Paleozoic marine sediments. Ordovician units are also present and form sediment layers up to thick. About 14.5 million years ago volcanic activity started in the region, first west of Galán but by 7 million years ago it shifted to the future caldera, forming the Cerro Colorado, Pabellon and Cerro Toconquis composite volcanoes on its future western rim. The more westerly centres are today represented by eroded volcanoes. Since about 6.6 million years ago the volcanic activity produced rocks of both mafic and silicic compositions. The increase of volcanic activity has been attributed to the steepening of the Nazca Plate slab which allowed mantle material to penetrate into the space between the lower crust and the slab. North of 21° degrees southern latitude ignimbritic volcanism started earlier, generating the Altos de Pica and Oxaya formations. Mafic volcanism occurred south and west of Galán both